An air/fuel ratio control system for an internal combustion engine is becoming increasingly important with respect to the control of noxious emissions from the engine. In such a system, engine parameters such as intake air flow rate, engine rotational speed and engine temperature are detected for determining the air/fuel ratio. Moreover, if the system is equipped with a feedback system, a gas sensor is provided in order to detect the concentration of a component contained in the exhaust gases where the sensor output is utilized for precisely regulating the air/fuel ratio of the air-fuel mixture supplied to the engine.
The fuel supplying means for an internal combustion engine is usually a carburetor or an injection system. In the case of a carburetor, the fuel flow rate is basically determined by the magnitude of the vacuum in the venturi disposed in the intake manifold. However, in an injection system, an air flow meter is usually employed for detecting the flow rate of the intake air and producing a signal indicative thereof, this signal being used to control the fuel flow rate through the injection system. While such an air flow meter is essential in the injection system it can also be advantageously employed with a carburetor to precisely modify the air/fuel ratio of the air-fuel mixture producing therein.
An air flow meter consists of a rotatable or pivotal flap disposed in the intake passage where the flap is mechanically connected to a movable contact of a potentiometer. The flap is arranged to rotate against the biasing force of a spring under the influence of the pressure difference on the upstream side of the flap and the downstream side of same. The potentiometer is arranged to produce an output signal the voltage of which is indicative of the angular displacement of the flap and which is utilized for control of the air/fuel ratio control system.
In such an air flow meter, a damper or a damping device is employed for reducing the flactuation of the movement of the flap. However, when the air flow rate increases abruptly, the movement of the flap is apt to be excessive to produce an overshoot phenomena and thus the potentiometer connected thereto produces an output signal indicative of an air flow rate which is higher than the actual air flow rate. This erroneous signal causes the air/fuel ratio control system to supply a higher rate of fuel flow than necessary so that the air-fuel mixture becomes richer than a predetermined or desired value. Although a closed loop type air/fuel ratio control system is basically advantageous for avoiding undersirable influences of engine parameters, the closed loop system is easily influenced by such an erroneous signal since a time delay is inherent therein. The undesirably enriched air-fuel mixture causes an increase of the concentration of toxic components in the exhaust gases and also a decrease in the efficiency of a catalytic converter (if a three-way type), if disposed, in the exhaust system since such a catalytic converter exhibits its maximum efficiency when the air/fuel ratio of the air-fuel mixture is within a narrow range (usually close to the stoichiometric value). Such an overshoot of the flap of the air flow meter also occurs when the intake air flow rate decreases abruptly and thus the potentiometer produces an erroneous signal in the same manner.
The above mentioned undesirable overshoot characteristics of the flap of the air flow meter can be reduced to a negligible extent by designing and adjusting the damper or the damping device carefully and precisely. However, such an air flow meter requires a complex construction and time consuming adjustment of same. Therefore, the above mentioned provision of a complex damper for the reduction of the overshoot characteristics causes an increase in the cost of the air flow meter.